Make serde_test build script buildable with older rustc

serde_test/build.rs

@@ -1,6 +1,6 @@
 use std::env;
 use std::process::Command;
-use std::str;
+use std::str::{self, FromStr};
 
 // The rustc-cfg strings below are *not* public API. Please let us know by
 // opening a GitHub issue if your build environment requires some way to enable
@@ -19,12 +19,30 @@ fn main() {
 }
 
 fn rustc_minor_version() -> Option<u32> {
-    let rustc = env::var_os("RUSTC")?;
-    let output = Command::new(rustc).arg("--version").output().ok()?;
-    let version = str::from_utf8(&output.stdout).ok()?;
+    let rustc = match env::var_os("RUSTC") {
+        Some(rustc) => rustc,
+        None => return None,
+    };
+
+    let output = match Command::new(rustc).arg("--version").output() {
+        Ok(output) => output,
+        Err(_) => return None,
+    };
+
+    let version = match str::from_utf8(&output.stdout) {
+        Ok(version) => version,
+        Err(_) => return None,
+    };
+
     let mut pieces = version.split('.');
     if pieces.next() != Some("rustc 1") {
         return None;
     }
-    pieces.next()?.parse().ok()
+
+    let next = match pieces.next() {
+        Some(next) => next,
+        None => return None,
+    };
+
+    u32::from_str(next).ok()
 }